Paraneurons (parafollicular cells), found in the thyroid gland, are endocrine cells, but developmentally related to neurons and moreover can be readily isolated in a pure form for experimental analysis. Serotoninergic neurons cannot similarly be isolated. Work on this cell preparation has revealed that the secretion competent granule interiors experience transient pH drops after cell stimulation. Chloride channels that are regulated by protein kinases C and G are implicated. We are simultaneously imaging 5-HT and a granule pH marker to investigate proposed correlations between 5-HT release and granule pH.